German philosopher Immanuel Kant’s Critique of Pure Reason is one of the most influential texts in the history of modern philosophy. Situated in the intellectual milieu of 18th century Europe, the Critique of Pure Reason is a philosophical document of the Age of Enlightenment and offers an answer to the philosophical debates of its day touching on metaphysical, epistemological, and ethical inquiries. Written in 1781 and substantially revised in 1787, Kant’s Critique inaugurated a philosophical tradition now known as German Idealism, which includes such other German luminaries as Fichte, Schelling, and, most notably, Hegel.

Among other things, the Critique of Pure Reason functions as a sustained introduction to Kant’s philosophy of critical idealism, a system of ideas that seeks to determine the limits of metaphysical knowledge via a “science” of human reason. It is the business of the critique to set determinant boundaries to all possible knowledge through an investigation into the nature of human experience. This is part of a larger Kantian project known as transcendental idealism, a philosophy that seeks the necessary conditions for the possibility of objective reality by investigating the powers of the human mind.

The Critique of Pure Reason is the first of three critiques. The second, The Critique of Practical Reason, followed in 1788, and the third, The Critique of Judgment, in 1790. This summary relies on the Hackett edition of the Critique of Pure Reason as translated from the German by Werner S. Pluhar. All citations refer to this edition.

Summary

Just as Copernicus revolutionized astronomy with the adoption of a heliocentric model of the solar system (thereby displacing the old geocentric model), Kant hoped to revolutionize metaphysics by displacing and reversing an age-old assumption in philosophical perspective. Instead of asking how the faculties of the human mind conform to the nature of objects (as his predecessors had), Kant asks how objects must conform to the nature of the mind. The mind took center stage as the determining ground for the possibility of metaphysical knowledge and the investigation into the sources of human experience.

Rather than accept the empiricist perspective that the mind is a blank slate that passively receives and associates sense data, Kant investigates the structure of consciousness, dividing the faculties of the mind into (for starters) sensibility and understanding. Sensibility is that faculty of the mind that receives information from the senses. It is the original source for all information/sensation given to consciousness from the outside world. Sensibility is constituted by space and time, the outer and inner sense that serve as the ideal limits through which all sense data must be presented to the mind. The faculty of understanding, on the other hand, deals in concepts—thoughts of objects produced by the mind. Though its content is provided by sensation, the conceptual work of the understanding occurs within the autonomous, non-experiential realm of the mind’s own activity. Together, these faculties produce objects of experience. Sensibility provides the material from the outside world and understanding imposes a form (or structure) on that material. Much of Kant’s work concerns how sensibility and understanding work together (along with imagination) to produce human experience.

For Kant, an essential question grounding the entirety of his inquiry regards how synthetic a priori propositions are possible. This question is fundamental for the possibility of legitimizing metaphysics as a serious discipline. Kant makes a distinction between pure and empirical cognitions. Empirical cognitions are based on the receptive capacity of sensory experience. Pure cognitions are provided by the spontaneous activity of the mind through the faculty of understanding. All empirical cognitions are a posteriori—or derived from experience. Pure cognitions, on the other hand, are a priori; that is, they proceed from reason independent of experience. Kant makes a similar distinction between analytic and synthetic judgments. Analytic judgments are made when the concept is judged as identical to itself, but in synthetic judgments something additional is added to the concept from outside of itself. For metaphysics to gain the legitimacy and limits it requires, it must proceed, Kant writes, through synthetic a priori propositions. Kant’s answer to this highly technical, but imminently practical, question forms the basis of the Critique.

Though the language, subject material, and method of the Critique of Pure Reason is dense and highly academic, Kant’s purpose is practical. Ultimately, he is concerned with human autonomy, i.e., self-governance, or freedom. It is only based on such autonomy that morality is possible. His related concerns about God and the nature of the soul are also practically relevant. Kant attempts to outline the absolute boundaries of possible knowledge in order, as he says, “to make room for faith” (31). Much of this work is done in the “Transcendental Dialectic,” a division of the Critique in which Kant shows how human reason falls into a series of unfortunate antinomies, or contradictory views, that it cannot resolve through itself. Historically, philosophers across the rationalist and empiricist divide have succumbed to the fallacious reasoning at the heart of the antinomies. Kant attempts to prove that direct metaphysical knowledge of God, amongst other things, is impossible.